Function returns $true if v1 only contains characters found in v2

/*
Usage $isonly(v1,v2)

Case sensitive version:
$isonly(v1,v2).cs

For example:
$isonly(hello,abcdefghijklmnop) returns $true
$isonly(hello,abcdefg) returns $false
$isonly(Today is 2005,abcdefghijklmnopqrstuvwxyz 1234567890) returns $true
$isonly(Today is 2005,abcdefghijklmnopqrstuvwxyz 1234567890).cs returns $false
*/

alias isonly {
  var %a = 1
  while %a <= $len($$1) {
    if $mid($$1,%a,1) $+(!isin,$prop) $$2 return $false
    inc %a
  }
  return $true
}